Heat exchangers are known which are constituted by a tank with a vertical axis in which is arranged a descending flow of a charge of granular material between a feed opening and a discharge opening placed at its upper and lower ends, the charge constituting a mobile bed which moves from above downwards in contact with an elongated exchange member extending inside the tank between two levels, respecitively below and above the latter. The exchange member may be immersed in the charge or may cover the inner wall of the tank and may be constituted, for example, by a tube or a bundle of tubes traversed by a heat transfer fluid.
To facilitate heat exchange, it has been proposed to inject at the base of the tank a gas distributed homogeneously inside the charge and which, even with a relatively small flow rate, forms an ascending current passing through the charge from below upwards and capable of fluidising the latter as soon as the speed of flow is sufficient. It is thus possible to considerably increase the heat transfer coefficient of the exchanger.
Generally, the heat exchange coefficient of such an exchanger is relatively constant and, to vary the power of the exchanger, the practice has developed, for example, of partitioning the fluidized bed into compartments whose number is adjusted according to the desired power. It is thus possible to vary the height of the charge or its speed of downward flow as well as the flow rate of the heat transfer fluid, but the adjustments are not instantaneous and can be made only in limited fashion. Now, in certain applications, it may be useful to vary very rapidly and to a relatively great extent, the heat exchange coefficient of the exchanger, for example to maintain the output temperature of the materials or the rise in temperature of the heat transfer fluid constant whatever the flow rate of the materials.